On-line sensors to assess the chemical characteristics of fluid streams are in common use. Calibration and recalibration of such chemical sensors has typically been accomplished off-line, for example, by placing a pH electrode in separate buffer solutions of known pH. Such off-line calibration suffers from a number of drawbacks, including lack of automation and, in some cases, accuracy due to electrode hysteresis. Current methods of on-line pH calibration typically utilize buffered solutions of known pH and generally high ionic strength. Low ionic strength buffers are not used because they are not stable. The problem with the use of such high ionic strength solutions for on-line calibration is that due to their high concentration of ions, an unacceptably long period of time is required for the pH electrode to regain its sensitivity and thus its measuring or sensing capacity, often referred to as "recovery time". In the case of measurement of low ionic strength sample solutions, this recovery time problem is especially egregious as the recovery time is especially long (often a matter of hours) due to the very large ionic strength differential between the calibrating buffered solution and the solution whose ionic strength is to be determined. U.S. Pat. No. 4,713,618 discloses an on-line system for calibrating pH sensors comprising the use of a solution of concentrated acid or base to condition the fluid to a predetermined pH level. Shortcomings of such a conditioning solution include the same recovery time problem mentioned above, the relatively large volume of solution needed and the need for a diffusion chamber.
There is therefore a need in the chemical sensor calibration art for a simple, reliable, space-saving calibration standard that may be used both on-line and off-line and that permits quick recovery of a chemical sensor's capacity to measure. These needs and others, which will be apparent to those skilled in the art, are met by the present invention, which is summarized and described in detail below.